Recently, magnetic recording media have been widely used as video tapes, audio tapes, magnetic tapes for computers and in other comparable tape materials. In such applications, the magnetic recording medium is required to be excellent in various characteristics such as electromagnetic conversion characteristics and running durability. In particular, adequate running characteristics as well as excellent magnetic conversion characteristic are both very important requirements, and it is necessary to optimize both characteristics in a compatible manner.
In order to meet both required characteristics, a magnetic disk has been proposed which is intended to prevent outputs due to surface properties and to improve durability at the same time by forming pores having an average pore area of 6.times.10.sup.-5 to 7.times.10.sup.-1 .mu.m.sup.2 (pore diameter: 86 .ANG. to 9560 .ANG.) in the surface of a magnetic layer by random orientation of the magnetic layer, and by further impregnating the pores with a lubricant as described in JP-A-62-137718 (the term "JP-A" as used herein means an "unexamined published Japanese patent application).
Following this development, a magnetic recording medium has been proposed in which pores contained in a coating layer have a pore diameter distribution of 0.01 to 0.1 .mu.m (100 to 1000 .ANG.) and occupy 70% by volume is proposed with an object of improving the durability thereby as described in JP-A-63-167416.
More recently, however, systems of high image quality and high sound quality such as S-VHS and S-8 mm have been evaluated, and further systems such as high density type high visions and floppy disks have been evaluated. Accordingly, the demand for increase in recording density is greater. In order to achieve the increase of the density, it is necessary to make the surface of a magnetic layer extremely smooth, for example, to a center line average roughness (Ra; cut-off value: 0.25 mm) (according to JIS B 0601) of 10 nm or less, preferably 5 nm or less. Since such an extremely smooth surface increases the coefficient of friction encountered by the running tape, further improvements in running durability are required.
To satisfy such a demand, the average pore areas described in JP-A-62-137718 and JP-A-63-167416 described above are too large to obtain sufficient smoothness. Further, the sustaining effect of the lubricants is reduced because of the large inlet radius of the pores. Namely, in the prior-art magnetic recording media, the surfaces of the magnetic layers are not adequately smoothly formed, and, as a consequence, the sliding area of the magnetic layers on magnetic heads is relatively small. As a result, major problems are not encountered in sliding characteristics. However, such relatively unsmooth surfaces are not improved in S/N in ultra wavelength, and, therefore, excellent electromagnetic conversion characteristics can not be obtained. Also, the prior-art magnetic recording media have the drawback that the lubricants easily ooze or seep from the pores, because the inlet radius of the pores is relatively large, and, as a result, the lubricating effect cannot be sustained over a prolonged period of time.
Further, JP-A-62-22239 discloses that running stability and running durability are improved by adding a fatty acid to a magnetic layer and by extracting the fatty acid in an amount of 5 to 30 mg per cm.sup.3 of magnetic layer from the surface of the magnetic layer with a non-polar hydrocarbon solvent. However, in the case of the magnetic recording media for high density recording having the extremely smoothed surfaces, even if such a so-called free fatty acid is contained in the magnetic layer, sufficient running stability and running durability can not be obtained.
Furthermore, JP-A-63-167413 discloses that fluorine-contained lubricants such as perfluoropolyethers can also be used. However, the ether linkage is relatively low in polarity and the inlet radius of the pores is relatively large, so that sufficient running stability and running durability can not be obtained.